Non-volatile memory devices, such as flash memory devices, are used to store data and other kinds of information. Many computers and electronic devices, for example, digital audio players, digital cameras, digital recorders, and cellular phones, have flash memory devices. Flash memory devices may also be used as portable storage devices, such as portable Universal Serial Bus (USB) flash drives or “thumb” drives. Flash memory devices may not need power to maintain the information stored in the device.
A flash memory device stores information in numerous memory cells, which are usually formed in a semiconductor chip. Each of the cells often has a metal-oxide semiconductor (MOS) transistor with two different transistor gates: a control gate and a so-called “floating” gate or FG. The control gate may be used to turn the transistor on and off to control access to the cell. The floating gate may be the place where either one bit or multiple bits of information is stored in each cell.
The value of the information stored in the floating gate may be determined by the amount of electrons or charge in the floating gate. Different amount of electrons in the floating gate may correspond to different values of information. The amount of electrons in the floating gate may be varied by either adding electrons to or extracting electrons from the floating gate.
A flash memory device usually has a programming operation (which is also sometimes referred to as a write operation) to add electrons to the floating gate, and an erase operation to extract electrons from the floating gate. Information in the cells may be read in a read operation. Each cell has a threshold voltage value dependent on the amount of electrons in the floating gate. A read operation determines the threshold voltage value of the cell being read to provide the value of the information stored in the cell.
In some cases, the physical distance among adjacent floating gates of adjacent cells, or the number of electrons in adjacent floating gates, or both, may create floating gate to floating gate (FG-FG) interference among adjacent cells. The FG-FG interference may cause the threshold voltage value of a cell to shift, leading to reduced reliability of information stored in the cell when the information is read.